Regulated apoptosis is critical to T cell development in the thymus and controls T cell- dependent adaptive immunity in periphery. 2-catenin, a coactivator of T cell factor 1 (TCF-1), and retinoid-related orphan receptor gamma t (ROR3t) both regulate thymocyte survival via the up-regulation of anti-apoptotic Bcl-xL. In the process of studying ROR3t, we have identified 2- catenin/TCF-1 as a potential upstream pathway that controls ROR3t-mediated thymocyte survival. Deletion of TCF-1 resulted in thymocyte apoptosis and down-regulated ROR3t, whereas transgenic expression of a stabilized 2-catenin (2-catTg), which activated TCF-1 constitutively, led to enhanced thymocyte survival and up-regulated ROR3t. In contrast to its survival role in thymocytes, 2-catTg up-regulated pro-apoptotic Bid and surface Fas, and enhanced super-antigen staphylococcal enterotoxin B (SEB)-induced deletion of peripheral T cells by promoting activation-induced cell death (AICD). We thus hypothesize that the 2- catenin/TCF pathway utilizes distinct mechanisms in the regulation of apoptosis in developing T cells and peripheral mature T cells. In the first two aims of this study, we propose to elucidate the mechanisms responsible for 2-catenin/TCF-1-regulated apoptosis in thymocytes and peripheral T cells. In the last aim, we will determine whether we can control T cell-dependent allograft rejection by manipulating 2-catenin-regulated T cell survival. Public Health Relevance: This proposal is to study the mechanisms responsible for 2-catenin and ROR3t-regulated T cell apoptosis.